DE69804375T2 - Synthesis gas production with ion transport membranes - Google Patents

Description

BACKGROUND OF THE INVENTION

Synthesis gas, which contains hydrogen and carbon oxides, is an important base product for the manufacture of a wide range of chemical products. Synthesis gas mixtures with the appropriate ratios of hydrogen to carbon monoxide are catalytically reacted to produce liquid hydrocarbons and oxygenated or enriched compounds, including methanol, acetic acid, dimethyl ether, oxo alcohols and isocyanates. High purity hydrogen and carbon monoxide are recovered by further processing and separation of the synthesis gas. The cost of producing the synthesis gas is usually the largest part of the total cost of these products.

Two main reaction pathways are used to produce syngas, namely steam reforming of light hydrocarbons, primarily natural gas, naphtha and refinery off-gases, and partial oxidation of carbonaceous feedstocks ranging from natural gas to high molecular weight liquid carbonaceous materials or solid carbonaceous materials. Autothermal reforming is an alternative process using a light hydrocarbon-based feedstock that combines features of both partial oxidation and steam reforming in a single reactor. In the various versions of the process, the feed gas is partially oxidized in a specially designed burner and the resulting hot gases pass through a catalyst bed where steam reforming occurs. Newer processes for producing syngas include various heat exchanger reformers, such as gas heated reforming (GHR) developed by ICI, the SMART reformer by KTI and the CAR reformer by UHDE; the improved Texaco gasification process (TGP) process) contained in their HyTEXTM hydrogen production system; Haldor Topsoe's HERMES process; the Shell gasification process (SGP); Exxon's fluidized bed syngas process; and Kellogg's KRES process.

The state of the art for commercial technology in syngas production is summarized in representative review articles, including "Steam Reforming - Opportunities and Limits of the Technology" by J. Rostrup-Nielsen et al., presented at the NATO ASI Study on Chemical Reactor Technology for Environmentally Safe Reactors and Predictors, Aug. 25 - Sept. 6, 1991, Ontario, Canada; "Improve Syngas Production Using Autothermal Reforming" by T. S. Christiansen et al., in Hydrocarbon Processing, March 1994, pp. 39-46; "Evaluation of Natural Gas Based Synthesis Gas Production Technologies" by T. Sundset et al., in Catalysis Today, 21 (1994), pp. 269-278; "Production of Synthesis Gas by Partial Oxidation of Hydrocarbons" by C. L. Reed et al., presented at the 86th National AIChE Congress, Houston, Texas, April 1-5, 1979; "Texaco's HyTEXTM Process for High Pressure Hydrogen Production" by F. Fong, presented at the KTI Symposium, April 27, 1993, Caracas, Venezuela; and "Custom-Made Synthesis Gas Using Texaco's Partial Oxidation Technology" by P. J. Osterrieth et al., presented at the AIChE Spring National Congress, New Orleans, LA, March 9, 1988.

In the commercial partial oxidation processes described above, oxygen is required and is typically supplied at purities of 95 to 99.9 vol.%. Oxygen is obtained by the separation of air using known methods, usually cryogenic distillation. of air for larger volumes and pressure swing adsorption for smaller volumes.

An alternative technology for the production of syngas is in the early stages of development, whereby oxygen is provided for the reactions in the partial oxidation in situ by the decomposition of air at high temperatures using ceramic, ceramic-metal or ceramic-ceramic composite membranes that conduct both electronic species and oxygen ions. These membranes are part of a class of membranes generally referred to under the generic term "ion transport membranes" and are in a specific class of ion transport membranes that conduct both electronic species and oxygen ions, collectively known as mixed conducting membranes. These membranes can be used in combination with suitable catalysts to produce syngases in a membrane reactor without the need for a separate oxygen production step. The reactor is characterized by one or more reaction zones, each zone having a mixed conducting membrane that separates the zone into an oxidant side and a reactant side.

An oxygen-containing gas mixture, typically air, is contacted with the oxidant side of the membrane, and oxygen gas dissociates to form oxygen ions which diffuse through the membrane material. A reactant gas containing methane and other low molecular weight hydrocarbons, typically natural gas, optionally with the addition of steam, flows across the reactant side of the membrane. Oxygen on the reactant side of the membrane reacts with components in the reactant gas to form synthesis gas containing hydrogen and carbon monoxide. A catalyst to accelerate the mass transfer of oxygen in the membrane may be applied to the surface of the membrane on the oxidant side. A catalyst to accelerate the conversion of the components of the reactant gas to synthesis gas may be applied to the surface of the reactant side of the membrane; alternatively, a granular form of the catalyst may be applied in the near the membrane surface. Catalysts that promote or accelerate the conversion of hydrocarbons, steam and carbon dioxide are well known in the art.

Furthermore, numerous reactors and mixtures of mixed conducting membranes suitable for this purpose have been disclosed in the prior art. Membrane reactors and methods of operating these reactors for the selective oxidation of hydrocarbons are disclosed in related U.S. Patents 5,306,411 and 5,591,315. Ceramic membranes with wide ranges of compositions are described that promote or accelerate the mass transfer of oxygen from an oxygen-containing gas and the reaction of the transferred oxygen with a methane-containing gas to form synthesis gas. Mixed conductors having a single-phase perovskite structure are used for the membrane material; alternatively, multiphase solids are used as dual conductors, with one phase conducting oxygen ions and another conducting an electronic species. A membrane reactor for producing synthesis gas is disclosed which operates at a temperature in the range of 1000 to 1400°C, wherein the reactor can be heated to the desired temperature and the temperature can be maintained during the reaction by external heating and/or exothermic heat from the chemical reactions taking place. In a general embodiment, the process is disclosed to be carried out at temperatures in the range of 1000 to 1300°C. Experimental results are reported for oxygen flow and synthesis gas production in a laboratory isothermal reactor using a double conducting membrane at a constant temperature of 1100°C. Inert diluents such as nitrogen, argon, helium and other gases can be present in the reactor feed and do not interfere with the desired chemical reactions. If steam is present in the reactor feed, it is considered an inert gas or a diluent.

In a paper entitled "Ceramic Membranes for Methane Conversion" presented at the Coal Liquefaction and Gas Conversion Contractors, Review Conference, 7-8 Sept. 1994, Pittsburgh, PA., U. Balachandran et al. described the fabrication of long tubes made of Sr-Co0.5-Fe-Ox membranes and the operation of these tubes for the conversion of methane to synthesis gas in laboratory reactors at 850ºC.

US Patent 4,793,904 discloses the use of a solid electrolyte membrane with conductive coatings on both sides optionally connected by an external circuit. The membrane is used in an electrolytic cell at temperatures ranging from 1050 to 1300°C to convert methane to synthesis gas at a pressure of about 0.1 to about 100 atmospheres. Experimental results are presented for the conversion of methane to synthesis gas components in a reactor cell with an yttria-stabilized zirconia membrane with platinum electrodes, optionally using an external electrical circuit. The reactor cell was operated isothermally at a temperature of 800, 1000 or 1100°C.

Related U.S. Patents 5,356,728 and 5,580,497 disclose cross-flow electrochemical reactor cells and the operation of these cells to produce synthesis gas from methane and other light hydrocarbons. Mixed conducting membranes made from mixed oxide materials having perovskite or non-perovskite structures are disclosed for use in the cross-flow and cross-flow reactor cells, respectively. The production of synthesis gas by the partial oxidation of hydrocarbons is disclosed using reactor temperatures of about 1000 to 1400°C or, alternatively, in the range of about 450 to 1250°C. Experimental results are reported for the production of synthesis gas in isothermal, tubular laboratory reactors at constant temperatures in the range of 450 to 850°C. The pressure in the reactor with the ceramic tube, typically about 6 inches of water column, was maintained by means of a downstream water agitator (pneumatic), i.e. a downstream water bubbler.

US Patent 5,276,237 discloses the partial oxidation of methane to synthesis gas using a mixed metal oxide membrane containing alumina with multivalent activation metals such as yttrium and barium. A low oxygen recovery process concept is proposed. to facilitate heat removal and maintain a high driving force for the oxygen partial pressure. The partial oxidation reactions were carried out at a temperature in the range of about 500 to about 1200°C, and the temperature on the oxygen side of the membrane is said to be at most only a few degrees lower than the reaction temperature on the reactant side of the membrane.

The practical application of mixed conducting membranes to produce syngas will utilize reactor modules having a plurality of individual membranes with appropriate inlet and outlet flow tubes to transport the feed and product gas streams. Such modules provide the large membrane surface area needed to produce commercially viable volumes of syngas product. Various designs for membrane modules are disclosed in the prior art that address this requirement. U.S. Patents 5,356,728 and 5,580,497, previously cited, describe a type of cross-flow membrane reactor having hollow ceramic vanes positioned transversely to a gas stream flow or having a stack of transverse hollow ceramic vanes containing channels for the gas stream. Alternatively, the cross-flow reactor can be fabricated in the form of a monolithic core with appropriate inlet and outlet tubes. US Patent 4,791,079 discloses membrane module designs for mixed conducting membrane reactors for the oxidative coupling of methane to produce higher hydrocarbons, hydrogen and carbon oxides.

A planar membrane module is described in EP 0 732 138 A2 which includes a plurality of planar units each comprising a channel-free porous support with an outer layer of a mixed conducting oxide material. An oxygen-containing gas is passed through the porous supports and the permeate oxygen reacts with the light hydrocarbons on the outer layer of the mixed conducting oxide material. The module is heated to a temperature ranging from about 300 to 1200°C for the continuous production of synthesis gas. US Patent 5 599 383 discloses a tubular solid state membrane module with a plurality of mixed conducting tubes, each containing an internal porous material that supports the tube walls and permits gas flow within the tube. The module may be used to produce synthesis gas by passing an oxygen-containing gas through the inside of the tubes and a hydrocarbon-containing gas over the outside of the tubes. The module is heated to a temperature ranging from 300 to 1200°C, with the oxygen-containing gas passing through the tubes while the hydrogen-containing gas passes over the outside of the tubes. Oxygen diffuses through the mixed conducting tube walls and reacts with the hydrocarbon under controlled conditions to produce synthesis gas containing hydrogen and carbon monoxide. A catalyst to promote the formation of synthesis gas may be applied to the outer surface of the tubes.

The prior art in this area summarized above characterizes the temperatures and pressures in mixed conducting membrane reactors for syngas production in general, non-spatial terms, i.e., differences in temperature and pressure as a function of reactor geometry are not taken into account. All of the above disclosures teach operation of reactors at a single temperature, i.e., as isothermal reactors, particularly for laboratory-scale reactors. In some cases, general temperature ranges for reactor operation are disclosed, but no information is offered regarding how temperature varies with reactor geometry. In all cases, gas pressures are reported as single pressures independent of geometry, and no pressure differences are disclosed between the oxidizer (air) side and the hydrocarbon (fuel) side.

C.-Y. Tsai et al. describe a non-isothermal, two-dimensional computational model for a mixed conducting membrane reactor using a perovskite membrane for the partial oxidation of methane to syngas. This work is reported in related publications entitled "Simulation of a Nonisothermal Catalytic Membrane Reactor for Methane Partial Oxidation to Syngas" in the Proceedings of the Third International Conference on lnorganic Membranes, Worcester MA, 10-14 July, 1994 and "Modeling and Simulation of a Nonisothermal Catalytic Membrane Reactor" in Chem. Eng Comm., 1995, volume 134, pages 107-132. The simulation describes the effects of gas flow rate, reactor length and membrane thickness on methane conversion and syngas selectivity for a tubular reactor configuration with air on the shell side. Temperature profiles as a function of axial reactor position are also presented. Key parameters are held constant for all simulation cases; in particular, the pressure on both the shell and tube sides of the reactor is specified as 1 atm, and the inlet temperature is specified at 800ºC. Additional discussion of the experimental and computational work on subjects in these two publications is presented in the PhD thesis of C.-Y. Tsai entitled "Perovskite Dense Membrane Reactors for the Partial Oxidation of Methane for Synthesis Gas", May 1996, Worcester Polytechnic Insitute (available through UMI Dissertation Services).

The practical application of mixed conducting membranes to produce synthesis gas requires reactor modules with a plurality of individual membranes with appropriate inlet and outlet flow lines to transport feed and product gas streams. Successful operation of such reactor modules will require careful selection and control of inlet, intermediate and outlet gas temperatures, as these temperatures affect both the chemical reactions occurring in the reactor and the mechanical integrity of the reactor assembly. In addition, gas pressures in the reactor will affect product distribution, reactor integrity, gas compression equipment and power requirements; therefore, gas pressures must be carefully specified in the design and operation of the reactor modules. The prior art has not addressed these important design and operating details. The production of synthesis gas using mixed conducting membranes Reactors will also involve the integration of reactor modules with feed gas supply systems and with product gas treatment and separation systems. The integration of mixed conducting membrane reactors into the overall process design for the production of synthesis gas has also not yet been addressed in the prior art.

The successful design and operation of synthesis gas generation systems using mixed conducting membrane reactors must include the specification of temperatures and pressures in the reactors and also the integration of the reactors with the gas processing systems provided upstream and downstream of the reactors. The invention described below and defined in the following claims addresses these practical design and operating requirements for synthesis gas generation in membrane reaction systems.

BRIEF SUMMARY OF THE INVENTION

The invention relates to a process for the production of synthesis gas containing hydrogen and carbon monoxide, which comprises:

(a) providing a reaction zone having an oxidant side and a reactant side separated by a solid mixed conducting membrane;

(b) heating an oxygen-containing oxidant gas feed and introducing the resulting heated oxidant gas feed into the oxidant side of the reaction zone at an oxidant gas feed temperature and an oxidant gas feed pressure;

(c) heating a methane-containing reactant gas and introducing the resulting heated reactant gas feed into the reactant side of the reaction zone at a reactant gas feed temperature and a reactant gas feed pressure;

(d) permeating oxygen from the oxidant side of the reaction zone through the mixed conducting membrane to the reactant side of the reaction zone and reacting the oxygen with the methane-containing reactant gas to form at least hydrogen and carbon monoxide;

(e) withdrawing the synthesis gas product comprising at least hydrogen and carbon monoxide from the reactant side of the reaction zone at a product gas outlet temperature;

(f) maintaining the reactant gas feed temperature at between 950ºF (510ºC) and 1400ºF (760ºC) and the product gas outlet temperature at greater than 1500ºF (815ºC),

(g) wherein the total pressure at any point on the reactant side of the reaction zone is greater than the total pressure at any point on the oxidant side of the reaction zone.

In a preferred embodiment, the oxidant gas feed pressure is between 1 and 45 psig (0.069 to 3.1 barg). The reactant gas feed pressure is typically between 100 and 900 psig (6.9 to 62 barg), and the oxidant gas feed temperature can be up to 200°F (111°C) greater than the reactant gas feed temperature.

Oxidant gas dehydrogenated is withdrawn from the oxidant side of the reaction zone at an oxygen-depleted oxidant gas temperature equal to or less than the product gas outlet temperature and the oxidant gas feed temperature is less than the oxygen-depleted oxidant gas temperature. In a preferred embodiment, at least 90% of the oxygen in the heated oxidant gas feed permeates to the oxidant side of the reaction zone, the mixed conducting membrane. If desired, a catalyst can be used on the reactant side of the reaction zone to promote or accelerate the formation of hydrogen and carbon monoxide.

The methane-containing reactant gas may additionally comprise one or more components selected from the group consisting of water, carbon dioxide and hydrogen. In a preferred embodiment, the methane-containing reactant gas contains water and the water:carbon molar ratio is in a preferred embodiment between 0.5 to 5, wherein the water:carbon molar ratio is defined as the number of water molecules in the reactant gas divided by the total number of carbon atoms contained in the hydrocarbons and carbon monoxide in the reactant gas.

In a particular embodiment of independent claim 1, the invention is a process for the production of synthesis gas containing hydrogen and carbon monoxide, additionally comprising one of the following steps:

(f) withdrawing deoxygenated oxidant gas from the oxidant side outlet of the reaction zone;

(g) at least a portion of the heat for heating the oxygen-containing oxidant gas in step (b) or for heating the methane-containing reactant gas in step (c) or for heating both the oxygen-containing oxidant gas in step (b) and the methane-containing reactant gas in step (c) is provided by indirect heat exchange with the deoxygenated oxidant gas from the reaction zone.

According to claim 1, the outlet temperature of the product gas is greater than the reactant gas feed temperature. The oxygen-containing oxidant gas in step (b) may be heated by direct combustion with a fuel in a direct-ignition or direct-burning combustion unit to produce a hot, pressurized combustion product that provides the heated oxidant gas feed. The oxygen-containing oxidant gas is optionally heated by indirect heat exchange with the oxygen-depleted oxidant gas from the reaction zone.

The total pressure at any point on the reactant side of the reaction zone is greater than the total pressure at any point on the oxidant side of the reaction zone. Typically, the oxidant gas feed pressure is between 1 and 45 psig (0.069 to 3.1 barg) and the reactant gas feed pressure is between 100 and 900 psig (6.9 to 62 barg).

The reactant gas feed temperature should be between 950ºF (510ºC) and 1400ºF (760ºC) and the product outlet temperature is greater than 1500ºF (815ºC). The oxidant gas feed temperature can be up to 200ºF (111ºC) greater than the reactant gas feed temperature, and the oxygen-depleted oxidant gas temperature can be equal to or less than the product gas outlet temperature. The oxidant gas feed temperature is typically less than the oxygen-depleted oxidant gas temperature, and in a preferred embodiment approximately 90% of the oxygen in the oxygen-containing oxidant gas permeates the oxygen-permeable membrane. A catalyst may be used on the reactant side of the reaction zone to promote or accelerate the formation of hydrogen and carbon monoxide. The synthesis gas product withdrawn from the reactant side of the reaction zone may be cooled to a temperature below 800ºF (427ºC).

The methane-containing reactant gas may comprise one or more components selected from the group consisting of water, carbon dioxide and hydrogen. In a preferred embodiment, the methane-containing reactant gas comprises water and the water:carbon molar ratio is between 0.5 and 5, wherein the water:carbon molar ratio is defined as the number of water molecules in the reactant gas divided by the total number of carbon atoms present in the hydrocarbons and carbon monoxide in the reactant gas.

If desired, the syngas product withdrawn from the outlet of the oxidant side of the reaction zone is quenched by direct contact with liquid water to reduce the gas product temperature below the dew point of the resulting gas mixture, and a hot water stream is separated from the uncondensed syngas components. The methane-containing reactant gas can be obtained by contacting a methane-containing gas feed with this hot water stream or some other heated hydrogen, thereby introducing water into the methane-containing reactant gas.

The synthesis gas product typically contains carbon dioxide, and the synthesis gas product can be cooled, dehydrated and at least a portion of the carbon monoxide removed therefrom. At least a portion of the resulting removed carbon dioxide can be pre-incorporated into the methane-containing reactant gas on the reactant side of the reaction zone.

At least a portion of the synthesis gas product is optionally separated to yield a hydrogen-enriched gas stream and an offgas stream, and at least a portion of the hydrogen-enriched gas stream is introduced into the methane-containing reactant gas prior to the reactant side of the reaction zone. The oxygen-containing reactant gas may be heated at least partially prior to the reaction zone by direct combustion with at least a portion of the offgas stream as fuel in a direct-ignition combustion unit to produce a hot, pressurized combustion product that provides the heated oxidant gas feed.

BRIEF DESCRIPTION OF THE DIFFERENT VIEWS OF THE DRAWINGS

Fig. 1 is a process flow diagram of the process according to the present invention.

Fig. 2 is a qualitative plot of normalized reactant volume or mass gas temperature versus cumulative membrane area or amount of oxygen permeated for a mixed conducting membrane reactor.

Figure 3 is a process flow diagram of a specific embodiment of the process of the present invention.

Fig. 4 is a process flow diagram of another specific embodiment of the process according to the present invention.

Figure 5 is a graph of methane conversion, hydrogen/carbon monoxide ratio, oxygen moles permeated per mole (hydrogen+CO), and carbon dioxide moles produced per mole (hydrogen+CO) versus reactor outlet temperature of Example 2.

Fig. 6 is a plot of methane conversion, hydrogen/CO ratio, oxygen moles permeated per mole (hydrogen+CO), carbon dioxide moles produced per mole (hydrogen+CO), and carbon dioxide moles recycled per mole (hydrogen+CO) versus reactor outlet temperature for Example 3.

DETAILED DESCRIPTION OF THE INVENTION

The present invention defines methods and processes for operating a mixed conducting membrane reactor module for producing syngas by the controlled reaction of hydrocarbons with oxygen, wherein oxygen is provided in situ by permeation of an oxygen-containing gas through the mixed conducting membrane. The reactor module is integrated with process steps for supplying reactants (in one embodiment, air and natural gas) and process steps for removing and further treating reactor effluents (in one embodiment, syngas and oxygen-depleted air). Preferred operating conditions are defined for the feed gas and product gas temperatures as well as for the pressure differential across the membrane in the reactor module.

The invention defines important operating conditions that have not previously been addressed or taken into account in the state of the art for mixed conducting high temperature membrane reactors.

A general embodiment of the invention is shown in Fig. 1. An oxygen-containing gas 1, in a preferred embodiment air, is pressurized in a compressor 3 to a pressure in the range of 1 to 45 psig (0.069 to 3.1 barg), preferably less than 10 psig (0.69 barg). Although air is the preferred oxygen-containing gas, other oxygen-containing gases may be used as the oxygen source for the process. The pressurized oxygen-containing gas 5 is preheated in the heat exchanger 7 in a heat transfer zone 9, and the preheated oxygen-containing gas 11 is further heated by direct combustion with air 14 in a burner 15 to yield heated oxidant 17 typically containing 15-20 volume percent oxygen at a temperature above 932°F (500°C) and in a preferred embodiment within ±200°F (111°C) of the reactant feed temperature. The burner 15 represents any type of known commercially available combustion device for effecting substantially complete combustion of the fuel 14 in an oxygen-excess environment.

The term "oxygen" is used here to refer generally to any form of oxygen (O, = , atomic number 8) present in the gas streams and the reactor systems described. The generic term oxygen also includes the molecular form of two oxygen atoms (O₂), oxygen ions (e.g. O⁻ or O= ), atomic oxygen (O·), or other forms of oxygen derived from the molecular diatomic oxygen in the gas streams and the systems described. The term oxygen as used here does not include oxygen chemically bound in carbon oxides, nitrogen oxides, or other oxygen-containing compounds.

The gaseous methane-containing hydrocarbon stream 19 is maintained at a pressure of 100-900 psig (6.9-62.1 barg), in a preferred embodiment 200-400 psig (13.8-27.6 barg), optionally by compression or depressurization (not shown) of a source gas. The methane-containing stream 19 may be a methane-containing gas from a petroleum refinery, petrochemical plant or other industrial source, or it may be a natural gas obtained from a pipeline or directly from the well after suitable pretreatment. The methane-containing stream 19 may be a natural gas having a typical composition in the range of at least 80 vol.% methane, less than 20 vol.% ethane, less than 10 vol.% propane, less than 5 vol.% alkanes having more than 3 carbon atoms, less than 10 vol.% carbon dioxide, less than 10 vol.% nitrogen and less than 100 ppmv total sulfur.

The methane-containing stream 19 is optionally combined with a hydrogen stream 21 and optionally heated in a heat exchanger 23 in a heat transfer zone 9 to a temperature up to 800°F (427°C). The resulting heated stream is optionally passed through a desulfurization/hydrogenation reaction zone 25 containing a hydrogenation catalyst typically comprising cobalt and molybdenum or nickel and molybdenum. In the reaction zone 25, olefinic hydrocarbons are converted to paraffins and organic sulfur compounds are converted to hydrogen sulfide which is sorbed onto a zinc oxide layer in the reactor. Typically, a reactor vessel containing the hydrogenation catalyst is operated in series with two reactors filled with zinc oxide and operated in parallel (not shown), with one online and the other being regenerated. This method of removing olefins and sulfur components is a well-known practice in steam methane reforming of natural gas to prevent coke formation (from olefin cracking) and catalyst poisoning in the reforming reactor.

The treated methane-containing gas 27 (which may also contain residual hydrogen) is optionally combined with steam 29 and/or carbon dioxide 31 and the combined stream is heated to 950 to 1400°F (510 to 760°C) in the heat exchanger 33 in the heat transfer zone 9 to yield the heated reactant 35. In a preferred embodiment, steam is used and the water:carbon molar ratio is between about 0.5 to about 5, wherein the water:carbon molar ratio is defined as the number of water molecules in the heated reactant 35 divided by the total number of carbon atoms contained in the hydrocarbons and carbon monoxide in the heated reactant 35.

The heated oxidant 17 and the heated reactant 35 are introduced into the mixed conducting membrane reaction zone 37. The gas at the oxidant inlet 39 is at a temperature of at least 932°F (500°C) and preferably at least ±200°F (111°C) of the temperature of the heated reactant gas at the reactant inlet 41. The gas temperature at the reactant inlet 41 is in the range of about 950 to 1400°F (510 to 760°C).

The mixed conducting membrane reaction zone 37 is shown schematically with an oxidant side 43 separated from the reactant side 45 by the mixed conducting membrane 47 and is presented in simplified format for the following description of reactor operation. The oxidant side 43 represents a reactor volume through which oxidant gas flows and is in contact with the oxidant side surface of the mixed conducting membrane 47. At this surface, diatomic oxygen is ionized to form oxygen ions and the oxygen ions permeate through the mixed conducting membrane 47 to the reactant side surface of the membrane.

The term "mixed conducting membrane" as defined herein defines a solid material or mixture of solid materials that simultaneously conducts both oxygen ions and electronic species (e.g., electrons). The mixed conducting membrane may comprise any solid material or materials known in the art that perform these simultaneous functions. These materials are described, for example, in U.S. Patent No. 5,306,411 cited above and in an article entitled "Electropox Gas Reforming" by T. J. Mazanec in Electrochem. Soc. Proceedings 95-24, 16 (1997).

Alternatively, the mixed conducting membrane may be a mixture of one or more ion-conducting solid materials and one or more solid materials that conduct electronic species (such as electrons), the mixture of the solid materials forming a mixed conducting composite membrane. One example of a mixed conducting composite membrane uses zirconium oxide as the oxygen ion-conducting solid material and palladium as the conductor for electronic species. Another example of a mixed conducting composite membrane uses zirconium oxide as the oxygen ion-conducting solid material and a mixture of indium and praseodymium oxides as the conductor for the electronic species.

The active mixed conductive membrane material in the mixed conductive membrane 47 may be a thin layer on a planar or tubular porous support, as is known in the art. The support may be made of an inert material that does not conduct oxygen ions and/or electronic species under the operating conditions of the process. Alternatively, the support may be an ionically conductive material, an electronic species conductive material, or a mixed conductive oxide material of the same or different composition as the active layer of the mixed conductive membrane material. In a preferred embodiment, the porous support is made of a material having thermal expansion properties compatible with the mixed conductive membrane material, and the compounds forming the respective layers should be selected from materials that do not adversely chemically react with one another under the operating conditions of the process.

The surface of the mixed conductive membrane 47 on the oxidizing side 43 can optionally be coated with a catalytic material to accelerate or promote the transfer of oxygen into the membrane. These materials are known in the art and include metals and metal oxides selected from groups 2, 5, 6, 7, 8, 9, 10, 11, 15 and the F-block lanthanides of the Periodic Table of the Elements according to the International Union of Pure and Applied Chemistry.

Suitable metals include platinum, palladium, ruthenium, gold, silver, bismuth, barium, vanadium, molybdenum, cerium, praseodymium, cobalt, rhodium and manganese.

The reactant side 45 represents a reactor volume through which the reactant gas flows and reacts with oxygen permeated through the mixed conducting membrane 47. On the reactant side 45, a series of chemical reactions occur among the various chemical components present including oxygen, hydrogen, water, carbon monoxide, carbon dioxide, methane and possibly elemental carbon. The primary reactions are shown below.

CH&sub4; + ¹/&sub2;O&sub2; 2H2 + CO (1)

CH&sub4; + 3/2 O&sub2; 2H2O + CO (2)

CH&sub4; + 2O2 2H2 O + CO2 (3)

CH4 + H2 O 3H2 + CO (4)

CH&sub4; + CO2 2H2 + 2 CO (5)

CO + H2 O H2 + CO2 (6)

H&sub2; + CO C + H2 O (7)

2CO CO₂ (8)

CnHm nC + m/2 H₂ (9)

Reactions similar to reactions (1) to (5) above occur with heavier hydrocarbons such as ethane and propane, if present.

These reactions are similar to the known reactions that occur in the conventional partial oxidation of methane to product synthesis gas. The oxidation reactions (1), (2) and (3) as shown above consume diatomic molecular oxygen that can occur on the reactant side 45 of the membrane reaction zone 37. In addition, other forms of oxygen as described above can react with methane (and other hydrocarbons) to form carbon oxides. The exact reaction mechanisms between the permeated oxygen and the hydrocarbons on the reactant side 45 are not yet fully understood, but carbon monoxide and carbon dioxide are formed as reaction products. Reactions (1), (2), (3) and (6) are exothermic reactions, while reactions (4) and (5) are endothermic reactions; the exothermic reactions (2) and (3) are kinetically very fast, require some form of oxygen, and can occur without any catalyst; in contrast, the endothermic reactions 4 and 5 are slower and use the reforming catalyst. If the local oxygen flux is too high and the endothermic reactions cannot kinetically keep up with the exothermic reactions, the region will heat up too much. The local oxygen flux and the associated volume and activity of the catalyst must be matched to allow the endothermic reactions to occur to a level sufficient to prevent overheating of the region. The catalyst must be located close to the ion transport membrane on the reactant side to minimize heat and mass transfer resistance.

If the membrane reactor zone 37 is designed such that in any region on the reactant side 45 of the membrane reactor zone 37 the reactant gas is essentially in chemical equilibrium, the bulk gas temperature on the reactant side qualitatively follows a profile as shown in Fig. 2, which is a representative plot of the normalized local reactant gas bulk temperature versus the cumulative membrane area or alternatively the cumulative amount of permeated oxygen. Except for a possible initial decrease, the bulk gas temperature increases smoothly and monotonically. The reason for the possible The reason for this initial reduction is that reactions (4) and (5) can proceed even in the absence of oxygen flow if steam or carbon dioxide is present in the feed and the feed is far from its chemical equilibrium.

If sufficient membrane area is provided for oxygen permeation, the reactant bulk gas temperature can rise smoothly in relation to the cumulative membrane area, theoretically to any temperature up to the adiabatic flame temperature of the total reactor feed, if not affected by heat leaks from the membrane reactor. Such potential heat leaks should be minimized by appropriate insulation and reactor design. The reactor outlet temperature should be kept substantially lower than the adiabatic flame temperature, since at this temperature the product gas will contain only carbon dioxide and water because all the hydrogen and CO have been consumed by combustion.

Material limitations of both the reactor and the equipment downstream will also limit the reactor outlet temperature. If the reactor temperature is too low (less than about 1500ºF), the conversion of methane would be too low and the proportion of carbon dioxide relative to carbon monoxide would become high. Figure 5 illustrates these concepts for a feed with a steam:carbon ratio of 1.6 at 250 psig as described in Example 2 below. The carbon dioxide produced per mole of syngas goes through a minimum - at low outlet temperatures, catalytic carbon oxide convergence favors the existence of carbon dioxide, while at high outlet temperatures, the carbon monoxide combustion reaction favors the production of carbon dioxide. In addition, membrane conversion drops off sharply at low temperatures.

Methane and excess carbon dioxide are generally undesirable in the product gas, as these components reduce the partial pressure of the desired components hydrogen and carbon monoxide, increase the requirements for the Increased syngas compression, increased purge requirements of the downstream syngas consumption process, and resulted in the generation of excess fuel. The generation of excess carbon dioxide also represents a detrimental consumption of oxygen, requiring increased feed air blower capacity and increased reactor size unless the carbon dioxide is recycled for extinction. If carbon dioxide is removed from the syngas product, the size of the carbon dioxide removal system will be increased; if carbon dioxide is recycled, the size of the compressor for recycling the carbon dioxide and the power requirements will also be increased. The reactor outlet temperature should therefore be carefully specified, designed, constructed and controlled to minimize unwanted excess carbon dioxide.

Reactions (7), (8) and (9) form elemental carbon, which is undesirable for the operation of the reactor. The deposition of carbon, also known as "coking", can cause serious problems at the inlet of the reactor, in the reactor and in the outlet lines downstream of the reactor. Reaction (9) is known as hydrocarbon cracking, particularly the cracking of the higher hydrocarbons such as ethane, propane and butane which are present in natural gas at low but significant concentrations. Cracking is promoted by high temperatures and can occur over hot metal surfaces, at nickel catalyst sites and at acidic sites on refractory materials such as catalyst supports. The inlet piping for the reactants and the feed area for the membrane reaction zone 37 are particularly vulnerable to carbon deposition by this mechanism. The extent of carbon deposition by reaction (9) is controlled by the reactant feed temperature, composition and constituents, as well as by the gas pressure.

The presence of hydrogen under steam in the feed is beneficial to prevent carbon deposition. A mixture of natural gas and steam would typically be preheated to a temperature of 1022ºF (550ºC) A mixture containing methane, steam and hydrogen, but no hydrocarbons heavier than methane, could be heated to higher temperatures in excess of 1200ºF (649ºC) depending on the relative concentrations of the components. Once the reactant gas has entered the reaction zone 37 and begins to react, the heavier hydrocarbons rapidly disappear and a substantial amount of hydrogen is formed so that cracking becomes progressively less likely in the succeeding zones of the reactor. The gradual permeation of oxygen through the mixed conducting membrane 47 into the reactant is also beneficial in reducing the likelihood of carbon deposition.

The final heating of the reactant gas to the preferred reaction temperature range occurs rapidly in the membrane reaction zone 37 because the net reaction there is exothermic, as discussed above. Thus, an essential feature of the present invention is that the reactant gas is not fully preheated to the preferred temperature range above 1500°F (816°C) prior to the membrane reaction zone 37, but rather the reactant gas temperature rises in the reaction zone 37 as a reaction occurs there.

As the hot synthesis gas effluent 49 from the membrane reaction zone 37 cools in the downstream equipment, it will enter a temperature range where carbon deposition is favored by reaction (8), also known as the Boudouard reaction. The exact temperature depends primarily on the partial pressures of carbon monoxide and carbon dioxide in the stream. The carbon causes severe erosion by corrosion of metallic surfaces of the downstream heat transfer equipment, such as high temperature metallic alloys containing nickel; this is a phenomenon widely known as "metal dusting." The metal dusting is kinetically blocked below a temperature of 800ºF (427ºC). Thus, metal dusting can be avoided by maintaining all metallic surfaces downstream of the reactor at temperatures below 800ºF (427ºC). A process waste heat boiler achieves this by increasing the temperature of the metal pipes maintained near the temperature of boiling water. The heat flux and vapor fraction in the boiling water are limited so as to achieve high condensation heat transfer coefficients. Another approach is to quench the synthegas effluent 49 with a stream of warm water to a temperature below 800ºF (427ºC) prior to any heat exchange.

The total gas pressure at any point on the reactant side 45 is 100-900 psig (6.9-62 barg), preferably 200-400 psig (13.8-22.6 barg), and a small pressure drop occurs from inlet 41 to outlet 49. The total gas pressure at any point on the oxidant side 43 is in the range of 1 to about 45 psig (0.069-3.1 barg), in a preferred embodiment less than 10 psig (0.69 barg), and a small pressure drop occurs from inlet 39 to outlet 55. Thus, it is a preferred feature of the present invention that the total pressure at any point on the reactant side of the reaction zone is greater than the total pressure at any point on the oxidant side of the reaction zone, and this is preferred for the reasons discussed below.

Natural gas, typically used as the methane-containing gas for the process described above, is available through pipelines at industrial locations at 500-1200 psig. At production wellheads, it is available at 200 to 2000 psig, although about 1000 psig is typical. Gases from petroleum refineries are available at 60 psig or higher.

In the reactions discussed above, one mole of methane yields nearly one mole of carbon monoxide contained in approximately 3 moles of syngas withdrawn at approximately the pressure on the reactant side of the membrane reactor. The partial oxidation process typically requires approximately 0.6 moles of oxygen per mole of methane, requiring a minimum of approximately 3 moles of air at 100% oxygen recovery, and considerably more at lower recovery rates.

Air is available at ambient pressure. The compressor power required is approximately proportional to the molar flow rate and the logarithm of the pressure ratio. The cost of the compressor is related to the actual volumetric flow rate at inlet conditions; lower inlet pressures can increase compressor size and cost, even at the same molar flow rate. Compression ratios less than about 3 generally require only a single compression stage; higher ratios require additional stages with intercoolers.

The following general conclusions can be drawn based on the previous discussion:

a) it is preferred to compress the methane-containing gas rather than the air or the syngas product, since the flow rate of the methane-containing gas is the lowest and compression requires minimal energy or power; in some cases compression may even be omitted;

(b) compressing the product syngas is less desirable, primarily because it is produced at approximately three times the flow rate of the methane-containing gas feed; and

c) Compressing the air is the least desirable because it must be done at the highest flow rate and the air is available at ambient pressure.

Therefore, the membrane reactor should be designed to operate with the maximum pressure difference between the reactant side and the oxidant side, subject to reasonable mechanical and manufacturing constraints. The oxidant side should be operated as close to ambient pressure as possible to overcome the overall system pressure drop, the membrane reactor should be designed to minimize the pressure drop therein, and a blower or fan is preferably used to supply air to the reactor's oxidant preparation system.

As the oxidant and reactant gases flow through the membrane reaction zone 37, the oxygen permeates through the mixed conducting membrane 47 and reactions (1) through (6) occur on the reactant side 45 to yield the desired synthegas product. In a preferred embodiment, a reforming catalyst is applied to at least a portion of the reactant side surface of the mixed conducting membrane 47 to promote the desired reactions. Alternatively, a reforming catalyst in granular or pellet form can be packed into the reactant side 45 near the surface of the mixed conducting membrane 47. Catalysts for this purpose are known in the art.

Addition of steam 29 to the reactant gas 27 is highly desirable to moderate the temperature, prevent carbon deposition, or gasify all of the carbon that is formed and can serve as a reactant of the reactant side 45. Steam also minimizes the methane residue in the syngas product 51. The steam:carbon ratio is preferably between 0.5 and 5. By using a steam:carbon ratio of, for example, 3.5, the unreacted methane in the syngas product can be reduced to approximately 0.5 vol.% at 1650°F (899°C) at 250 psig (17.2 barg). Without the addition of steam, unreacted methane of 0.5 vol.% would only be achieved at temperatures approaching 2400°F (1315°C). Since the addition of steam increases the carbon dioxide/carbon monoxide ratio and reduces energy efficiency, the amount of steam added should be carefully specified.

Hot synthesis gas product 51 is withdrawn at outlet 49 of membrane reaction zone 37 at a temperature greater than 1500ºF (816ºC). Synthesis gas product 51 contains hydrogen and carbon monoxide with a hydrogen:carbon monoxide molar ratio of 1 to 6. Oxygen-depleted oxidant 53 is withdrawn at outlet 55 at a temperature below that of the product synthesis gas 51. When an oxidant and reactant flow in the same direction, the temperature of the oxygen-depleted oxidant 53 can approach the temperature of the product synthesis gas 51 to within a range of 9 to 180°F (5-100°C). The temperature rise in a controlled manner from the inlet to the outlet of the membrane reaction zone 37 is exothermic in net effect because of the combination of individual endothermic and exothermic reactions occurring therein, as described above.

In a preferred embodiment, at least 90% of the oxygen in the heated oxidant 17 permeates the mixed conducting membrane 47 so that the oxygen-depleted oxidant 53 generally contains less than 2% oxygen by volume. High oxygen recovery will minimize the power requirements of the compressor 3 because only a minimal volume of gas is compressed.

The mixed conducting membrane reaction zone 37 as described above is presented in a simplified format to illustrate the process characteristics of a membrane reactor. In actual practice, the mixed conducting membrane reaction zone 37 comprises one or more reactor modules, each containing a plurality of membranes with multiple oxidant and reactant channels or cells, with a single reaction cell characterized by the oxidant side 43, the reactant side 45 and the mixed conducting membrane 47 of Figure 1. Various designs of membrane reactor modules have been described for this purpose in the prior art as summarized in the background information above, and these designs include modules that utilize both cocurrent and crosscurrent flows, and tubular, corrugated plate and monolithic configurations may be employed.

Referring again to Fig. 1, the hot, oxygen-depleted oxidant 53 is introduced into the heat transfer zone 9 and exits therefrom as cooled effluent gas 57. A major portion of the heat content of the hot, Oxygen-depleted oxidant 53 is transferred through heat exchangers 7, 23 and 33 to heat process streams as described above. The heat transfer zone 9 may be similar to the exhaust heat recovery systems used in conventional steam methane reforming.

The hot syngas product 51 is rapidly cooled to a temperature below 800°F (427°C) against boiling water by indirect heat transfer in the waste heat boiler 59 and then further against other process streams (to be defined later) in one or more heat exchangers 61, 63, 65 and 67. The cooled syngas 69 enters a phase separator 71 from which condensed water 73 is withdrawn and combined with the boiler feedwater makeup 75. The combined water stream is heated in a heat exchanger 65 to yield preheated boiler feedwater 77 which is degassed and vented (not shown) for use in the waste heat boiler 59. Alternatively, if the process produces excess water, a portion of the condensate 73 is preheated in the heat exchanger 65 and the remainder is discharged as waste water (not shown). Depending on the final destination of the syngas, the cooled and dewatered syngas 79 is optionally treated in a carbon dioxide removal system 81 using known techniques to remove some or all of the carbon dioxide contained in the raw syngas. The processed syngas 85 is compressed in the compressor 87 as needed to yield the final syngas product 89.

Optionally, a portion of the carbon dioxide 83 removed from the raw syngas is compressed in compressor 91 to provide recycled carbon dioxide 31, as described above. Optionally, a portion 93 of the syngas 85 is separated using known techniques, such as in a pressure swing adsorption system 95, to recover hydrogen 21, which is used for hydrogenation of the feed gas 19, as described above. The exhaust gas 97 from the pressure swing adsorption system 95 may be combined with fuel 13 to produce fuel 14 in a Burner 15 for heating the oxygen-containing gas 11.

In an alternative embodiment of the invention, steam 29 is not used and instead the treated methane-containing gas 27 is directly saturated with water vapor prior to final heating and introduction into the mixed conducting membrane reaction zone 37. This alternative is illustrated in Figure 3, in which the treated methane-containing gas 27 is introduced into the saturation unit 201 where it is contacted with hot water 203 to achieve a water:carbon molar ratio of between 0.5 and 5. The saturation unit 201 may be any type of gas-liquid contact element such as a spray tower, a packed tower or a column with trays. The methane-containing gas 205, now containing the required amount of water, is optionally combined with the recycled carbon dioxide 31 and passed to the heat exchanger 33 as described above. The bottoms product of the saturator 207 is combined with the quench make-up water 209 and rapidly cools the hot syngas product 51 in the quench zone 211. The quenched gas and liquid water enter the separator 213 and the water withdrawn therefrom is returned to the saturator 201 by a pump 215. The quenched syngas 217 is further processed as described above.

In the embodiment of Figure 3, the latent heat for saturating the methane-containing gas 27 with water is obtained from the hot syngas product 51 using water as a "heat shuttle" and no external steam is required. This embodiment is useful when export steam has no value or when additional steam is not needed elsewhere in the syngas process. Thus, the boiler 59 of Figure 1 would not be used to generate steam and instead the embodiment of Figure 3 would be preferred.

A further embodiment of the invention is shown in Fig. 4, in which the methane-containing feed gas and the oxygen-containing gas are fed to alternative In this embodiment, the methane-containing gas 301 and reactant gas 303 are heated in a process furnace 305 fed with fuel 307 and combustion air 309 as is commonly practiced in autothermal reforming and partial oxidation syngas processes. The oxygen-containing gas 311, preferably air, is pressurized in a compressor 313 and heated against hot gas 317 (as will be defined later) in the gas-to-gas heat exchanger 315. The heated oxidant 319 and reactant gas 321 enter the mixed conducting membrane reaction zone 37 where syngas is formed as described above. The hot, oxygen-depleted oxidant 323 is optionally combined with cool, fresh air 325 to yield the hot, compressed gas 317. Cooling the hot, oxygen-depleted oxidant 323 with cool, fresh air or cool fresh air 325 enables the use of less expensive materials in the heat exchanger 315.

Thus, the present invention includes two preferred features in the operation of a mixed conducting membrane reactor module for the production of synthesis gas. First, the temperature of the hot synthesis gas product 51 as withdrawn at the outlet 49 of the membrane reactor zone 37 is greater than the temperature of the heated reactant gas 35 at the reactant inlet 41 of the reactant side 45 of the membrane reaction zone 37. In a preferred embodiment, the temperature of the reactant gas 35 is maintained below a threshold value which depends on the components in the reactant feed as well as on design and operating factors. The maximum value of this threshold temperature is 1400°F (760°C), but under certain conditions the preferred threshold temperature may be lower. For example, for a typical natural gas stream mixed with steam, this threshold temperature is preferably 1022°F (550°C). Maintaining the reactant gas feed temperatures below the appropriate threshold temperatures will prevent or minimize carbon formation from reaction (9) in the inlet line, distribution line and inlet region of the membrane reaction zone 37. Maintaining this inlet temperature below 1200ºF (635ºC) provides the additional practical Advantage that the gas inlet line to the reactor module can be made of unlined low alloy metal rather than a refractory lined metal, high alloy metal or ceramic material. This is a desirable design feature because unlined metal inlet piping can more easily compensate for the thermal expansion and contraction of the membrane reactor module components than inlet piping lined with refractory materials. Unlined metal inlet piping can be coiled to form "pigtails" that easily compensate for thermal effects at the reactor inlet. This keeps the reactant gas temperature below the threshold temperature and does not preheat it to the preferred reaction temperature range (greater than 1500°F (815°C)) prior to the membrane reaction zone 37. Thus, any heating of the reactant gas above the reactant unit temperature of 950 to 1400ºF (510 to 760ºC) occurs with the simultaneous chemical reaction within the membrane reaction zone 37.

The second preferred feature of the invention is that the gas pressure at any point on the oxidant side of the membrane reaction zone 37 is less than the pressure at any point on the reactant side of the zone 37. This is preferred for economic reasons because the synthesis gas product is normally required at an elevated temperature and the reactant, typically natural gas, is usually available at an elevated pressure and requires little or no compression prior to preheating. The pressure of the oxidant gas, preferably air, need only be sufficient to compensate for any pressure drop through the membrane reaction zone and associated piping and equipment. High pressure is not required to increase the partial pressure of oxygen because the rapid consumption of oxygen on the reactant side of the membrane reaction zone provides a sufficient oxygen partial pressure differential at the membrane. Also, as described above, at least about 90% of the oxygen in the heated oxidant 17 (preferably air) permeates the mixed conducting membrane 47, thereby minimizing the required flow rate of the oxidant gas. Making the required pressure and the flow rate of the oxidant gas is minimized, so the power or energy requirements for the compressor 3 are also minimized. Because of the lower pressure requirements, a blower can also be used as compressor 3, which can further reduce the capital costs.

EXAMPLE 1

The production of synthesis gas using the process of Figure 1 was modeled by heat and material balance calculations to illustrate an embodiment of the present invention. Air 1 is compressed to 10 psig (0.69 barg) in a blower 3, preheated in the heat exchanger 7 and the combustor 15, and the resulting combustion gas 17 at 1200°F (649°C) with an oxygen concentration of 17 vol% at a flow rate of 2240 lbmoles/hr (10170 kgmole/h) is introduced into the oxidant side 43 of the membrane reaction zone 37. The reactant mixture 27 at 285 psig (19.7 barg) containing 80 vol% methane and 20 vol% hydrogen at a flow rate of 6772 lbmoles/hr (3072 kgmole/h) is combined with 2701 lbmoles/hr (1225 kgmole/h) carbon dioxide 31 and 8668 lbmoles/hr (3932 kgmole/h) steam 29 to give a steam/carbon molar ratio of 1.6. A hydrogenation reactor 25 is not required and a pressure swing adsorption (PSA) system is not used. The resulting stream is heated in heat exchanger 33 and the heated reactant 35 at 1200°F (649°C) is passed to the reactant side 45 of the membrane reaction zone 37. The oxygen permeates through the mixed conducting membrane 47 and reacts with the reactant gas components to form synthesis gas as described above.

The oxygen-depleted air 53, containing 2 vol.% oxygen, is withdrawn at 1742ºF (950ºC), cooled in the heat exchange zone 9 and discharged as exhaust gas 57 at 289ºF (143ºC). The synthesis gas product 51 is withdrawn at 1742ºF (950ºC) and cooled to 750ºF (399ºC) in the waste heat boiler 59. After further cooling to 100ºF (38ºC) in various heat exchange zones steps as described above and after dewatering, 90% of the syngas 79 is processed in an MEA scrubber 81 to remove carbon dioxide 83 which is compressed to 285 psig (19.7 barg), recycled as stream 31 and combined with reactant 27. The remaining 10% of the syngas (not shown) is combined with the syngas 85 and compressed to 600 psig (41.4 barg) to provide the final syngas product 89 having a hydrogen/carbon monoxide molar ratio of 2.15 at a flow rate of 16249 lbmoles/hr (7370 kgmole/h). The product syngas contains 1.9 vol% carbon dioxide and 0.5 vol% methane.

EXAMPLE 2

A mixed conducting membrane reactor with a cocurrent flow configuration was simulated by equilibrium, heat and material balance calculations to show the performance or efficiency of the reactor as a function of outlet temperature. The reactor is operated in a cocurrent mode with an air-fuel combustion product as the oxidant gas and a methane-hydrogen vapor mixture entering the reactor as the reactant gas with a methane/hydrogen molar ratio of 4.0 and a steam/carbon molar ratio of 1.6. The inlet temperature for both the air and reactant is 1200ºF (649ºC), the air feed pressure is less than 10 psig (0.7 barg), and the syngas outlet pressure is 250 psig (17.3 barg). The mixed conducting membrane can be any membrane that is selectively permeable to oxygen at these temperatures. It is assumed that all of the oxygen passed through is to be consumed and it is further assumed that the synthegas product 49 is in the reforming and catalytic carbon monoxide shift equilibrium. Equilibrium constants for the steam reforming and catalytic carbon monoxide shift reactions are known in the art (see, for example, the brochure entitled "Physical and Thermodynamic Properties of Elements and Compounds" published by United Catalysts, Inc. or the Table 2 in the textbook entitled "Catalytic Steam Reforming" by Jens R. Rostrup-Nielsen, Springer-Verlag, 1984). Published equilibrium constants were curve-fitted and used in the simulation calculations.

The oxygen concentration in the oxygen-depleted oxidant gas from the reactor is 2 vol. The oxygen-depleted oxidant gas outlet temperature and the reactant gas outlet temperature are assumed to be substantially equal. It is also assumed that no heat loss from the reactor occurs, so that adiabatic operation is realized. Heat and mass balance calculations were performed at selected outlet temperatures and the results are presented in Figure 5, which gives methane conversion, hydrogen/carbon monoxide ratio, oxygen permeation and carbon dioxide production as a function of reactor outlet temperature. Temperatures above 2732ºF (1500ºC) are for illustrative purposes only and material limitations would make higher temperatures impractical.

It can be seen that methane conversion increases rapidly and is complete at just over 1000ºC. The hydrogen/carbon monoxide ratio decreases as temperature increases, dictated by the trend of reverse water gas shift or catalytic carbon monoxide shift equilibrium. The cumulative amount of oxygen passed through the membrane increases with temperature, and temperature increases because of the net exothermic chemical reactions occurring in the reaction zone which consume the permeated oxygen. Carbon dioxide production initially increases with temperature due to reverse carbon monoxide shift reaction equilibrium, and increases slowly and then more rapidly after a minimum at about 1100ºC. The rapid increase in carbon dioxide production at higher temperatures occurs because essentially all of the methane has been converted, and the products hydrogen and carbon monoxide begin to burn as additional oxygen has passed through the membrane. This continues until all of the combustible components are consumed and the temperature reaches the adiabatic flame temperature. (not shown in Fig. 5). The adiabatic flame temperature is a characteristic of the combination of reactant feed 41 and oxidant feed 39.

These results demonstrate the use of controlled oxygen permeation and high oxygen recovery to produce syngas, with the membrane reactor operated at conditions where the pressure on the reactant side is greater than the pressure on the oxidant (air) side.

EXAMPLE 3

This example represents operation of the membrane reactor at a constant hydrogen/carbon monoxide ratio with carbon dioxide recycling. All other conditions, assumptions and calculation methods are identical to those of Example 2. Heat and mass balance calculations were performed at selected outlet temperatures and the results are shown in Fig. 6, which plots menthane conversion, hydrogen/carbon monoxide ratio, oxygen permeation, carbon dioxide recycling and carbon dioxide production as a function of reactor outlet temperature.

Carbon dioxide is removed from the syngas product and recycled to the reactor feed and the amount recycled is adjusted to the extent possible to give a H₂/CO molar ratio of 2.15. This syngas is a suitable feed for, for example, the Fischer-Drip process for the synthesis of higher hydrocarbons. A controlled molar ratio of 2.15 is possible at reactor exit temperatures between 815ºC and 1399ºC. Below 815ºC, the H₂/CO molar ratio cannot be reduced to 2.15 even if all the carbon dioxide is recycled. Above 1399ºC, even without carbon dioxide recycling, the H₂/CO ratio cannot be increased to 2.15. The amount of carbon dioxide recycled decreases continuously with increasing temperature. The removal and recycling of carbon dioxide is costly. If it is desirable to minimize or eliminate carbon dioxide recycling, reactor exit temperatures will increase to the range of 1300-1400ºC. However, oxygen permeation requirements also increase monotonically as an increasing amount of carbon dioxide is produced in the reactor at the expense of carbon monoxide production.

Any excess amount of carbon dioxide produced that is not recycled will remain in the product synthesis gas, which is also undesirable, as it is a waste of permeated oxygen and also of carbon atoms in the methane-containing feed. In addition, it increases the purge requirements of the downstream processing section, and it increases the overall energy export of the plant as steam or fuel gas. Thus, the optimum reactor outlet temperature is probably somewhat lower than 1300ºC. Methane conversion is complete at 1000ºC, as in Example 2. Below about 816ºC, a significant amount of unconverted methane would remain in the synthesis gas product, which generally has the same disadvantages as described above for carbon dioxide in the synthesis gas product. Thus, the optimum reactor outlet temperature for this particular case with a S/C ratio of 1.6 is between about 816ºC and 1300ºC. Other steam/carbon ratios will have different optimal ranges - higher S/C ratios would generally shift the desired temperature to a lower range.

The present invention thus discloses the production of synthesis gas from a methane-containing feed gas using a mixed conducting membrane reactor operated at preferred conditions of temperature and pressure. Two preferred features are described for the operation of a mixed conducting membrane reactor module for synthesis gas production. First, the temperature of the hot synthesis gas product as withdrawn at the outlet of the membrane reaction zone is greater than the temperature of the heated reactant feed gas at the reactant inlet of the membrane reaction zone. According to a preferred In one embodiment, the reactant gas feed temperature is maintained below a threshold temperature which depends on the components in the reactant feed and on design and operating factors. The maximum value of this threshold temperature is approximately 1400°F (760°C), but the preferred threshold temperature may be lower under certain situations, as described above. The threshold temperature is less than the maximum temperature on the reactant side of the reaction zone. Maintaining the reactant gas feed temperature below the threshold temperature prevents carbon deposition and enables the use of unlined metal piping and elbows at the reactor inlet.

The second preferred feature of the invention is that the gas pressure on the oxidant side of the membrane reaction zone is less than the gas pressure on the reactant side. This is preferred for economic reasons because the synthesis gas product must normally be at an elevated pressure and the reactant, typically natural gas, is usually available at an elevated pressure and requires little or no compression prior to preheating. The pressure of the oxidant gas, preferably air, only needs to be sufficient to compensate for the pressure drop across the membrane reaction zone and associated piping and equipment. High pressure is not needed to increase the oxygen partial pressure because the rapid oxygen consumption on the reactant side of the membrane reaction zone provides a sufficient oxygen partial pressure difference across the membrane. By minimizing the required pressure and flow rate of the oxidizer gas, the power requirements for compression are minimized. Additionally, because of the lower pressure requirements, a blower can be used instead of a compressor, thereby reducing capital costs.

The essential characteristics of the present invention are fully described in the above disclosure. Those skilled in the art can understand the invention and can make various modifications, without departing from the spirit of the invention and from the scope and equivalents of the claims that follow.

Claims (15)

1. Process for the production of a synthesis gas product containing hydrogen and carbon monoxide, comprising: a) providing a reaction zone (37) having an oxidant side (43) and a reactant side (45) separated by a solid mixed conductive membrane (47); b) heating (7, 15) an oxygen-containing oxidant gas feed (5, 11) and introducing the resulting heated oxidant gas feed (17) into the oxidant side (43) of the reaction zone (37) at an oxidant gas feed temperature and an oxidant gas feed pressure; c) heating (23) a methane-containing reactant gas (19) and introducing the resulting heated reactant gas feed (35) into the reactant side (45) of the reaction zone (37) at a reactant gas feed temperature and a reactant gas feed pressure; d) permeating oxygen from the oxidant side (43) of the reaction zone (37) through the mixed conducting membrane (47) to the reactant side of the reaction zone (37) and reacting the oxygen with the methane-containing reactant gas to form at least hydrogen and carbon monoxide; e) withdrawing the synthesis gas product containing at least hydrogen and carbon monoxide from the reactant side (45) of the reaction zone (37) at a product gas outlet temperature; f) maintaining the reactant gas feed temperature between 950ºF (510ºC) and 1400ºF (760ºC) and the product gas outlet temperature greater than 1500ºF (815ºC), g) wherein the total pressure at any point in the reactant side (45) of the reaction zone (37) is greater than the total pressure at any point in the oxidant side (43) of the reaction zone (37). 2. The method of claim 1, wherein the oxygen-depleted oxidant gas is withdrawn from the outlet of the oxidant side (43) of the reaction zone (37); and wherein at least part of the heat for heating the oxygen-containing oxidant gas in step b) and/or the methane-containing reactant gas in step c) is provided by indirect heat exchange (7, 29) with the oxygen-depleted oxidant gas (53) from the reaction zone (37). 3. The process of claim 1 or 2 wherein the temperature of the oxidant gas feed is up to 200°F (111°C) greater than the temperature of the reactant gas feed. 4. The process of any one of claims 1 to 3, further comprising withdrawing the deoxygenated oxidant gas from the oxidant side (43) of the reaction zone (37) at a temperature of the deoxygenated oxidant gas equal to or less than the outlet temperature of the product gas. 5. The process of any one of claims 1 to 4, wherein at least 90% of the oxygen in the heated oxidant gas feed to the oxidant side (43) of the reaction zone (37) permeates the mixed conducting membrane (47). 6. A process according to any one of claims 1 to 5, wherein a catalyst is used on the reactant side (45) of the reaction zone (37) to promote the formation of hydrogen and carbon monoxide. 7. A process according to any one of claims 1 to 6, wherein the methane-containing reactant gas contains water, and wherein the molar ratio of water to carbon is between 0.5 to 5, wherein the molar ratio of water to Carbon is defined as the number of water molecules in the reactant gas divided by the total number of carbon atoms present in the hydrocarbons and carbon monoxide in the reactant gas. 8. A process according to any one of claims 1 to 7, wherein the outlet temperature of the product gas is greater than the temperature of the reactant gas feed. 9. A method according to any one of claims 1 to 8, wherein the oxygen-containing oxidant gas in step b) is heated by direct combustion with a fuel in a directly fired combustor (15) to produce a hot, pressurized combustion product which provides the heated oxidant gas feed. 10. The method according to any one of claims 1 to 9, wherein the oxygen-containing oxidant gas is heated by indirect heat exchange (7) with oxidant gas from the reaction zone (37) from which oxygen has been removed. 11. A process according to any one of claims 1, 2, 4 to 10, wherein the temperature of the oxidant gas feed is lower than the temperature of the oxidant gas from which oxygen has been removed. 12. The process of any one of claims 1 to 11, wherein the synthesis gas product withdrawn from the reactant side (45) of the reaction zone (37) is cooled to a temperature below 800°F (427°C). 13. The process of any one of claims 1 to 12, wherein the synthesis gas product withdrawn from the outlet of the oxidant side (43) of the reaction zone (37) is quenched by direct contact with liquid water to reduce the temperature of the gas product to below 800º F (427º C). 14. A method according to any one of claims 1 to 13, wherein the methane-containing reactant gas (35) is obtained by using a methane-containing feed Gas (19) is brought into contact with a heated water stream (29), whereby water is introduced into the methane-containing reactant gas. 15. The process of claim 14, wherein the heated water stream is obtained by contacting the synthesis gas product with liquid water to reduce the temperature of the gas product below the dew point of the gas product, and separating the resulting condensate therefrom.